Microelectronic imagers with shaped image sensors and methods for manufacturing microelectronic imagers

ABSTRACT

Microelectronic imagers with shaped image sensors and methods for manufacturing curved image sensors. In one embodiment, a microelectronic imager device comprises an imaging die having a substrate, a curved microelectronic image sensor having a face with a convex and/or concave portion at one side of the substrate, and integrated circuitry in the substrate operatively coupled to the image sensor. The imaging die can further include external contacts electrically coupled to the integrated circuitry and a cover over the curved image sensor.

TECHNICAL FIELD

The present invention generally relates to microelectronic imagers withshaped image sensors and methods for forming shaped image sensors foruse in such microelectronic imagers.

BACKGROUND

Microelectronic imagers are used in digital cameras, wireless deviceswith picture capabilities, and many other applications. Cell phones andPersonal Digital Assistants (PDAs), for example, incorporatemicroelectronic imagers for capturing and sending pictures. The growthrate of microelectronic imagers has been steadily increasing as theybecome smaller and produce better images with more pixels.

Microelectronic imagers include image sensors that use Charged CoupledDevice (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS)systems, or other solid-state systems. CCD image sensors have beenwidely used in digital cameras and other applications. CMOS imagesensors are also quickly becoming very popular because they are expectedto have low production costs, high yields, and small sizes. CMOS imagesensors can provide these advantages because they are manufactured usingtechnology and equipment developed for fabricating semiconductordevices. CMOS image sensors, as well as CCD image sensors, areaccordingly “packaged” to protect delicate components and to provideexternal electrical contacts.

FIG. 1 is a schematic side cross-sectional view of a conventionalmicroelectronic imaging unit 1 including an imaging die 10, a chipcarrier 30 carrying the die 10, and a cover 40 attached to the chipcarrier 30 and positioned over the die 10. The imaging die 10 includesan image sensor 12 and a plurality of bond-pads 16 operably coupled tothe image sensor 12. The chip carrier 30 has a base 32, sidewalls 34projecting from the base 32, and a recess defined by the base 32 andsidewalls 34. The die 10 is received within the recess and attached tothe base 32. The chip carrier 30 further includes an array of terminals18 on the base 32, an array of contacts 24 on an external surface 38,and a plurality of traces 22 electrically connecting the terminals 18 tocorresponding external contacts 24. The terminals 18 are positionedbetween the die 10 and the sidewalls 34 so that wire-bonds 20 canelectrically couple the terminals 18 to corresponding bond-pads 16 onthe die 10.

One problem with the microelectronic imaging unit 1 illustrated in FIG.1 is that the die 10 must fit within the recess of the chip carrier 30.Dies having different shapes and/or sizes accordingly require chipcarriers configured to house those specific types of dies. As such,manufacturing imaging units with dies having different sizes requiresfabricating various configurations of chip carriers and significantlyretooling the manufacturing process.

Another problem with conventional microelectronic imaging units is thatthey have relatively large footprints. For example, the footprint of theimaging unit 1 in FIG. 1 is the surface area of the base 32 of the chipcarrier 30, which is significantly larger than the surface area of thedie 10. Accordingly, the footprint of conventional microelectronicimaging units can be a limiting factor in the design and marketabilityof picture cell phones or PDAs because these devices are continuallybeing made smaller in order to be more portable. Therefore, there is aneed to provide microelectronic imaging units with smaller footprints.

The imager 1 shown in FIG. 1 also has an optics unit including a support50 attached to the chip carrier 30 and a lens system with a plurality oflenses 70 (identified individually by reference numbers 70 a–c).Traditional lens systems include a plurality of lenses for focusing theimage at the image sensor 12. Traditional lens systems accordinglyflatten the field of the image at the image sensor 12 so that the imageis focused across the face of the image sensor 12. In the embodimentshown in FIG. 1, for example, the lens 70 c may flatten the image “I”across the face of the image sensor 12. In other conventional systems,one or more of the lenses 70 a–c can be combined into a singleaspherical lens that can focus and flatten an image.

Another problem with conventional microelectronic imaging units is thatlens systems with multiple lenses or more complex aspherical lenses arerelatively tall and complex. Conventional lens systems accordingly havehigh profiles, can be expensive to manufacture, and may be difficult toassemble. Therefore, it would be desirable to reduce the demands andcomplexity of lens systems in the manufacturing of microelectronicimagers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side cross-sectional view of a packagedmicroelectronic imager in accordance with the prior art.

FIG. 2 is a cross-sectional view illustrating one stage of fabricating aplurality of microelectronic imagers at the wafer level in accordancewith an embodiment of the invention.

FIG. 3 is a cross-sectional view illustrating a subsequent stage offabricating a plurality of microelectronic imagers at the wafer level inaccordance with an embodiment of the invention.

FIGS. 4A and 4B are schematic side cross-sectional views illustratingalternative embodiments of microelectronic imagers fabricated inaccordance with an embodiment of the invention.

FIG. 5 is a cross-sectional view illustrating an embodiment for formingcurved image sensors in microelectronic imagers in accordance with anembodiment of the invention.

FIG. 6 is a cross-sectional view illustrating an embodiment for formingcurved image sensors in microelectronic imagers in accordance withanother embodiment of the invention.

FIG. 7 is a cross-sectional view illustrating an embodiment for formingcurved image sensors in microelectronic imagers in accordance withanother embodiment of the invention.

FIG. 8 is a cross-sectional view illustrating a process for bending asubstrate to fabricate curved microelectronic imagers in accordance witha specific embodiment of the method shown in FIG. 7.

FIG. 9 is a cross-sectional view illustrating another embodiment forfabricating curved image sensors in accordance with the invention.

FIG. 10 is a cross-sectional view illustrating a device and method forfabricating curved image sensors in accordance with still anotherembodiment of the invention.

FIG. 11 is a cross-sectional view illustrating a device and method forfabricating curved image sensors in accordance with yet anotherembodiment of the invention.

FIGS. 12A and 12B are cross-sectional views illustrating a device and aprocess for fabricating curved image sensors in accordance with anembodiment of the invention.

FIGS. 13A–13D are cross-sectional views illustrating stages offabricating microelectronic imager devices with curved image sensors inaccordance with another embodiment of the invention.

FIGS. 14A and 14B are cross-sectional views illustrating stages of aprocess for fabricating microelectronic imager devices with curved imagesensors in accordance with another embodiment of the invention.

FIG. 15 is a graph illustrating dark current effects on microelectronicimager devices.

FIG. 16 is a cross-sectional view illustrating an aspect of formingcurved image sensors in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

A. Overview

The following disclosure describes several embodiments ofmicroelectronic imagers having shaped image sensors and methods forfabricating such microelectronic imagers at the wafer level and at theindividual die level. In one embodiment, a microelectronic imager devicecomprises an imaging die having a substrate, a curved microelectronicimage sensor having a convex and/or concave face at one side of thesubstrate, and integrated circuitry in the substrate operatively coupledto the image sensor. The imaging die can further include externalcontacts electrically coupled to the integrated circuitry and a coverover the curved image sensor. In particular embodiments, the curvedimage sensor mitigates the extent that images need to be flattened sothat the images can be focused at the peripheral regions of the imagesensor.

The curved microelectronic image sensor can have a convex and/or concaveface with a desired radius of curvature. For example, the curved imagesensor can have a face with a single radius of curvature, a plurality ofcurves with different radii, and/or flat portions in combination withone or more curves. The curved face of the image sensor is expected toreceive a generally spherical image field such that the lens assemblydoes not need to significantly flatten the field to compensate for aplanar sensor array.

In an alternative embodiment, a microelectronic imager device includesan imaging die having a substrate with a bowed portion, amicroelectronic image sensor having a curved face at the bowed portionof the substrate, and integrated circuitry electrically coupled to theimage sensor. The imager device can further include a flexor unit thatexerts a force against the substrate to bend or otherwise flex thesubstrate to form the bowed portion under the image sensor. The flexorunit, for example, can include a first element attached to a firstregion of the substrate under an image sensor, a spacer attached to thesubstrate outwardly of the first element, and a plate attached to thefirst element and the spacer. The first element expands or contractsmore or less than the spacer to flex the substrate. The flexor unit canalternatively comprise a compartment at the front side and/or thebackside of the substrate and a fluid in the compartment at a pressurethat causes the substrate to bow. Another embodiment of the flexor unitcan comprise a material attached to the backside of the substrate thatbends the substrate into a desired curvature. The flexor unit canalternatively comprise an actuator attached to the backside of thesubstrate to flex the substrate and bend the image sensor into a desiredcurvature.

Another aspect of the invention is a method for manufacturingmicroelectronic imager devices. In one embodiment, such a methodincludes constructing an imaging die having a substrate, integratedcircuitry in the substrate, and an image sensor having a curved face atone side of the substrate. This method can further include positioning acover over the substrate and/or bending the substrate to flex the imagesensor.

Another aspect of the invention is directed toward a microelectronicimager device including an imaging die comprising a substrate having afront side and a backside, a microelectronic image sensor having a facelocated at the front side of the substrate, and integrated circuitryconnected to the image sensor. The imager device further includes abacking member including a shaped surface. In several embodiments, aportion of the backside of the substrate is attached to the shapedsurface of the backing member such that the face of the image sensor iscurved or otherwise contoured at least generally in the shape of theshaped surface.

Another aspect of the invention is directed toward a microelectronicimager device assembly. In one embodiment, the imager device assemblyincludes a substrate and a backing member. The substrate has a frontside, a backside, and a plurality of imaging dies arranged in a diepattern. The individual imaging dies comprise a microelectronic imagesensor located at the front side of the substrate and integratedcircuitry electrically coupled to the image sensor. The substrateaccordingly has a plurality of discrete image sensors and integratedcircuits. The backing member includes a plurality of curved surfacesarranged in the die pattern. Individual curved surfaces are aligned witha corresponding image sensor and attached the backside of the substrate.Several embodiments of the imager device assembly accordingly havecurved image sensors corresponding to the shape of the curved surfacesof the backing member.

Another aspect of the invention is directed toward a method ofmanufacturing microelectronic imager devices. One embodiment of such amethod includes providing a substrate having a front side, a backside,and a plurality of imaging dies arranged in a die pattern. The imagingdies comprise microelectronic image sensors located at the front side ofthe substrate and integrated circuitry electrically coupled to the imagesensors. The method further includes providing a backing membercomprising a plurality of curved surfaces arranged in the die pattern,and attaching the curved surfaces to the backside of the substrate suchthat the curved surfaces are aligned with corresponding image sensors.

Another embodiment of a method for manufacturing microelectronic imagerdevices comprises providing a substrate having a front side, a backside,and a plurality of imaging dies arranged in a die pattern. The imagingdies have image sensors and integrated circuitry connected tocorresponding image sensors. The method further includes providing amold having a plurality of shaped molding sites arranged in the diepattern, and conforming the imaging dies to the shaped molding sites tobend the image sensors to a desired curvature.

Several details of specific embodiments of the invention are describedbelow with reference to CMOS imagers to provide a thorough understandingof these embodiments. CCD imagers or other types of sensors, however,can be used instead of CMOS imagers in other embodiments of theinvention. Several details describing well-known structures oftenassociated with microelectronic devices may not be set forth in thefollowing description for the purposes of brevity. Moreover, otherembodiments of the invention can have different configurations ordifferent components than those described and shown in this section. Assuch, other embodiments of the invention may have additional elements ormay not include all the elements shown and described below withreference to FIGS. 2–16.

B. Microelectronic Imagers with Curved Image Sensors

FIG. 2 is a side cross-sectional view illustrating an imager unitassembly 200 having a plurality of microelectronic imager units 202 atone stage of a method for packaging imagers in accordance with anembodiment of the invention. The assembly 200 illustrated in FIG. 2includes an imager workpiece 210, standoffs 230 projecting from theimager workpiece 210, and a cover 240 attached to the standoffs 230. Aplurality of optics units (not shown) are typically mounted to the cover240 either before or after forming curved image sensors on the imagerworkpiece 210 to fabricate microelectronic imagers.

The imager workpiece 210 includes a substrate 212 having a front side214, a backside 216, and an initial thickness to between the front side214 and backside 216. The imager workpiece 210 further includes aplurality of imaging dies 220 formed on and/or in the substrate 212.Individual imaging dies 220 can include an image sensor 221, integratedcircuitry 222 operatively coupled to the image sensor 221, and terminals223 (e.g., bond-pads) electrically coupled to the integrated circuitry222. The image sensors 221 can be CMOS devices, CCD image sensors, orother solid state devices for capturing pictures in the visible spectrumor sensing radiation in other spectrums (e.g., JR or UV ranges). Asexplained in more detail below, the terminals 223 can be connected tothrough-wafer interconnects formed according to the processes disclosedin U.S. patent application Ser. No. 10/713,878 entitled “Methods forForming Vias in Microelectronic Devices, and Methods for PackagingMicroelectronic Devices,” filed on Nov. 13, 2003, which is incorporatedby reference herein in its entirety. Other embodiments of externalcontacts can include terminals that are at an intermediate depth withinthe first substrate 212 instead of being at the front side 214.

The embodiment of the imager unit assembly 200 illustrated in FIG. 2 isfabricated at the wafer level such that several imaging units 202 arepackaged before singulating (e.g., cutting) the first substrate 212, thespacers 230 and the cover 240 along lines A—A. One aspect of wafer-levelpackaging is using automated equipment to further process the assembly200 to form curved image sensors and to install optics units (not shown)onto the cover 240. FIGS. 3–4B illustrate several aspects of formingcurved image sensors and embodiments of assemblies having curved imagesensors.

FIG. 3 illustrates the imager unit assembly 200 at a subsequent stage ofa process for forming curved image sensors on the imaging dies 220. Atthis stage of the process, the substrate 212 has been thinned from theinitial thickness T₀ to a thickness T₁ so that the portions of thesubstrate 212 between the standoffs 230 are at least relativelyflexible. In several embodiments, the substrate 212 can be thinned usinga back grinding process, a chemical-mechanical planarization process,and/or an etching procedure known in the art to form a new backside 216.The final thickness T₁ between the front side 214 and the backside 216can be in the range of approximately 20–200 μm depending upon the typeof material. When the substrate 212 is composed of silicon, thethickness T₁ is generally less than approximately 150 μm and can be inthe range of approximately 20–80 μm. The very thin portions of thesubstrate 212 between the standoffs 230 acts much like a flexiblemembrane, and as such the portions of the substrate 212 under the imagesensors 221 can be flexed to bend the image sensors 221. After thinningthe substrate, the assembly 200 illustrated in FIG. 3 can be furtherprocessed to construct the through-wafer interconnects 224 through thesubstrate 212 to provide electrical contacts on the backside 216 of thesubstrate 212. Additional suitable processes for forming suchinterconnects are disclosed in U.S. application Ser. No. 10/879,838,entitled “Microelectronic Devices and Methods for Forming Interconnectsin Microelectronic Devices,” filed on Jun. 29, 2004, which is hereinincorporated by reference.

FIG. 4A is a cross-sectional view illustrating one embodiment of theimager unit assembly 200 after bending the substrate 212 to form curvedimage sensors 221. In this embodiment, the substrate 212 has curvedportions 250 in the areas aligned with the image sensors 221. The curvedportions 250 are generally discrete bowed regions of the substrate 212that form projecting bumps on the backside 216. In one embodiment, thecurved portions 250 have a shape of a portion of a sphere with a radiusof curvature R. The curved portions 250 are not limited to a sphericalconfiguration and can have other configurations with one or more curvesand/or flat portions depending upon the particular application.

The image sensors 221 flex as the curved portions 250 of the substrate212 are formed such that the image sensors 221 have curved faces 260.The curvature of each curved face 260 is configured so that the array onthe curved face 260 is at a desired focal distance for the image. In theembodiment illustrated in FIG. 4A, the curved image sensors 221 haveconcave curved faces 260 relative to the direction of the radiation toaccommodate non-planar image fields.

The curved image sensors 221 with the curved faces 260 are expected to(a) reduce the complexity of fabricating lens systems and (b) increasethe options of lens systems that can be used with the imagers. Forexample, because the image sensors 221 have curved faces 260, the imagefield does not need to be flattened using optics to the same extent asimage fields need to be flattened for planar image sensors. This isexpected to eliminate the need for field flattening lenses in the opticsunits that are attached to the cover 240, or at least reduce thecomplexity of field flattening lenses. Therefore, the imaging dies 220illustrated in FIG. 4A reduce the constraints on lens designs such thatfewer lenses or less complex lenses can be used to reduce the cost offabricating microelectronic imagers.

The curved image sensors 221 illustrated in FIG. 4A are alsoadvantageous because they are particularly well-suited for miniaturecamera applications that require a wide-angle field of view and/or havea short focal distance. One problem with miniature cameras is that it isdifficult to adequately flatten the image field because the focaldistance between the lenses and the image sensors 221 is extremelyshort. As a result, images from conventional miniature cameras aretypically focused at the center but out of focus at the periphery. Thecurved image sensors 221 mitigate this problem because the periphery ofthe image sensors 221 is at, or at least closer to, the desired focaldistance of the image field. The curved image sensors 221 are alsoexpected to be very useful for megapixel wide-angle applications thathave longer focal distances for the same reason. Therefore, the curvedimage sensors 221 are further expected to provide better quality imagesfor miniature cameras or other applications that have a wide-angle fieldof view.

FIG. 4B is a cross-sectional view illustrating another embodiment of theimager unit assembly 200 having a plurality of imaging dies 220 withcurved image sensors 221. In this embodiment, the curved portions 250 ofthe substrate 212 project into the cavity between the cover 240 and thesubstrate 212. The curved portions 250 accordingly form small discretedimples on the backside 216 of the substrate 212 such that the imagesensors 221 have convex curved faces 260 relative to the direction ofthe radiation. As described above, the curved portions 250 can have theshape of a portion of a sphere having a radius of curvature R, but otherconfigurations may also be suitable.

C. Methods and Devices for Forming Curved Image Sensors

FIG. 5 is a cross-sectional view of an embodiment of fabricating curvedimage sensors using a plurality of flexor units 500 attached to thebackside 216 of the substrate 212. The flexor units 500 can bepositioned at each imaging die 220 or only at known-good imaging dies220 depending upon the particular application. The individual flexorunits 500 include a first element 510 attached to the backside 216 ofthe substrate 212 under a corresponding image sensor 221. The firstelements 510, for example, can be expansion/contraction members attachedto the substrate 212 at areas aligned with the central regions of thecorresponding image sensors 221. The individual flexor units 500 canfurther include a spacer 520 arranged outwardly from the first element510 and a plate 530 attached to the first element 510 and the spacer520. In one embodiment, the first elements 510 are made from a materialhaving a first coefficient of thermal expansion, and the spacers 520 aremade from a material having a second coefficient of thermal expansionless than that of the first elements 510. In other embodiments, thefirst elements 510 can be a shape memory metal, such as Nitinol, and thespacers 520 can be a substantially incompressible material.

The flexor units 500 operate by expanding/contracting the first elements510 either more or less than the spacers 520 to bend the substrate 212in the local regions under corresponding image sensors 221. For example,the flexor units 500 can be attached to the substrate 212 at an elevatedtemperature, and then the assembly can be cooled such that the firstelements 510 exert local forces (arrows F) that bend the substrate 212into the concave curved portions 250 (shown in dashed lines) similar tothose shown in FIG. 4A. The spacers 520 in this example contract lessthan the first elements 510 as they cool. Alternatively, the firstelements 510 can have a lower coefficient of thermal expansion than thespacers 520 such that the first element 510 exerts a force in theopposite direction to form convex curved portions similar to thoseillustrated in FIG. 4B.

FIG. 6 is a cross-sectional view illustrating another embodiment forfabricating curved image sensors in microelectronic imagers using aplurality of flexor units 600 attached to the backside 216 of thesubstrate 212 under corresponding imaging dies 220. In this embodiment,individual flexor units 600 include a, compartment 610 and a fluid inthe compartment 610 at a pressure that causes the substrate 212 to bow(not shown in FIG. 6) in a manner that flexes a corresponding imagesensor 221. In one embodiment, the compartments 610 can be attached tothe substrate 212 in a low pressure environment such that the pressureinside the compartments 610 is less than the pressure in chambers 620over the corresponding image sensors 221. The pressure differentialbetween the compartments 610 and the chambers 620 exerts a force. F₁that draws the portions of the substrate 212 under the image sensors 221into the compartments 610 to form curved portions (not shown) similar tothe concave curved portions 250 illustrated above with respect to FIG.4A. Alternatively, the compartments 610 can be attached to the substrate212 in a high pressure environment such that the pressure in thecompartments 610 is greater than the pressure in the chambers 620. Thissecond embodiment exerts a force F₂ against the substrate 212 to drivethe portions of the substrate 212 under the image sensors 221 into thechambers 620 to form a convex curvature on the image sensors 221 asillustrated above with respect to FIG. 4B. The pressure in thecompartments 610 can also be set by vacuuming or pressurizing thecompartments 610 using gas or fluid lines connected to the compartments610.

FIG. 7 is a cross-sectional view illustrating yet another embodiment forforming curved image sensors on the assembly 200 using flexor units 700attached to the backside 216 of the substrate 212 underneathcorresponding image sensors 221. In this embodiment, the flexor units700 can be a material that expands or contracts in a manner that bendsthe portions of the substrate 212 under the image sensors 221 into aconcave and/or convex curvature. The flexor units 700, for example, canbe an epoxy deposited onto the backside 216 of the substrate 212 andthen cured in a manner that causes the epoxy to contract. As the epoxycontracts, it is expected to bend the substrate 212 to form convexcurved portions similar to those illustrated above with respect to FIG.4B. The epoxy can be deposited in many configurations, including acircle, radial starburst pattern, or other suitable pattern. The flexorunits 700 can alternatively be small members of a shape memory alloythat assumes a desired configuration when it is in an operatingtemperature range. For example, the shape memory alloy may be attachedto the substrate 212 at a first temperature and then expand, contract orotherwise flex as it reaches an operating temperature range to bend thelocal regions of the substrate 212 under the image sensors 221 in amanner that forms concave and/or convex portions similar to thoseillustrated above with respect to FIGS. 4A or 4B.

FIG. 8 is a cross-sectional view illustrating yet another embodiment ofa flexor unit 700 having a first material 710 and a second material 720.The first material 710 typically has a higher coefficient of thermalexpansion than the second material 720. As such, when the flexor 700cools to an operating temperature range, the first material 710contracts by a greater extent (arrows C.sub.1) than the second material720 (arrows C.sub.2). The difference in contraction is expected to causethe flexor unit 700 to exert a downward force against the substrate 212to form a concave curved face 260 (illustrated in dashed lines), asillustrated above with respect to FIG. 4A. In one embodiment, the firstlayer 710 can be composed of aluminum and the second layer 720 can becomposed of Kovar to form a bimetallic plate.

FIG. 9 illustrates another embodiment for bending the image sensors 221to have curved faces with a desired curvature. In this embodiment,flexor units 900 are defined by sealed chambers over the image sensors221 and a fluid in the sealed chambers at a pressure P. The pressure ofthe fluid causes the substrate 212 to flex in the regions under theimage sensors 221 as shown in FIGS. 4A and 4B. In one embodiment, thecover 240 is assembled to the standoffs 230 in an environment at apressure higher than ambient pressure such that the pressure in thesealed chambers drives the portions of the substrate 212 under the imagesensors 221 outwardly to form the concave faces on the image sensors asillustrated in FIG. 4A. In an alternative embodiment, the cover 240 isassembled to the spacers 230 in an environment at a pressure lower thanthe ambient temperature such that the substrate 212 is drawn into thecompartments to form convex curved faces on the image sensors asillustrated in FIG. 4B.

FIG. 10 illustrates another embodiment for bending the image sensorsinto a desired curvature in accordance with the invention using aplurality of flexor units 1000 attached to the backside of the substrate212 under corresponding image sensors 221. In this embodiment, theindividual flexor units 1000 include a bracket 1002 attached to thebackside 216 of the substrate 212 and an actuator 1010 attached to thebracket 1002. The actuator 1010 can have a first end 1012 in contactwith the backside 216 of the substrate 212 underneath a central portionof a corresponding image sensor 221. The actuator 1010 can furtherinclude a second end 1014 attached to the bracket 1002 and a line 1016for transmitting electrical signals or carrying fluids to control theactuator 1010. In one embodiment, the actuator 1010 is a piezoelectricelement and the line 1016 is an electrically conductive wire that can becoupled to a control unit. In a different embodiment, the actuator canbe a bladder or other type of structure that can be expanded/contractedby adjusting a fluid pressure. In still another embodiment, the actuator1010 can be a pneumatic or hydraulic cylinder. In operation, theactuator 1010 moves upwardly to form a convex curved face on the imagesensor 221 (see FIG. 4B) or downwardly to form a concave curved face onthe image sensor 221 (see FIG. 4A). The actuators 1010 can also beoperated in real time while using an imaging unit to provide fineadjustment of the focus for wide-angle applications and otherapplications.

FIG. 11 illustrates still another embodiment for bending the imagesensors into a desired curvature. In this embodiment, a flexor unit 1100is defined by a transparent cover attached to the standoff 230 at anelevated temperature. The transparent flexor unit 1100 has a coefficientof thermal expansion greater than that of the substrate 212 such thatthe flexor unit 1100 contracts more than the substrate 212 as theassembly is cooled. The corresponding contraction of the flexor unit1100 causes the substrate 212 to bend as shown by arrows B to form aconcave curved face on the image sensor 221 as shown above with respectto FIG. 4A.

FIGS. 12A and 12B are cross-sectional views that illustrate stillanother embodiment for bending the image sensors into a desiredcurvature in accordance with the invention using curved flexor units1200 attached to the backside of the substrate 212. The flexor units1200 are vacuum cups having an opening 1202 and an interior surface 1204with a curvature corresponding to the desired curvature for the imagesensors 221. FIG. 12A illustrates the process before the substrate 212is bent to form the curved face on the image sensor 221. At this stage,there is a gap 1206 between the backside 216 of the substrate 212 andthe interior surface 1204 of the flexor unit 1200. To bend the substrate212, a vacuum is drawn through the opening 1202. Referring to FIG. 12B,the vacuum drawn through the opening 1202 draws the backside 216 of thesubstrate 212 against the interior surface 1204 of the flexor unit 1200.The backside 216 of the substrate 212 and/or the interior surface 1204of the flexor unit 1200 can be covered with an adhesive that adheres thebackside 216 of the substrate 212 to the interior surface 1204 of theflexor unit 1200. The flexor unit 1200 can further include interconnects1224 that contact the interconnects 224 to carry the backside electricalcontacts from the substrate 212 to the exterior surface of the flexorunit 1200.

FIGS. 13A–13D illustrate various stages of an embodiment formanufacturing microelectronic imager devices at the wafer-level. Theprocesses illustrated in FIGS. 13A–13D are similar to the embodimentillustrated in FIGS. 12A–B and can use the assembly 200 illustrated inFIG. 3. Therefore, like reference characters refer to like components inFIGS. 3, 12A–B, and 13A–D.

FIG. 13A is a cross-sectional view illustrating an, early stage of amethod for manufacturing a plurality of microelectronic imager devicesat the wafer-level. At this stage, the assembly 200 is aligned with abacking member 1300 such that the substrate 212 faces a plate 1310 ofthe backing member 1300. The plate 1310 can be a mold having a pluralityof flexor units 1320. The individual flexor units 1320 can be moldingunits similar to the flexor units 1200 described above with respect toFIGS. 12A–B. In this embodiment, the flexor units 1320 include shapedsurfaces 1322 defined by depressions or concave areas along the plate1310. The shaped surfaces 1322 can be curved surfaces that have acurvature corresponding at least generally to the desired curvature forthe image sensors 221. The flexor units 1320 can further include one ormore openings 1324 terminating at the shaped surfaces 1322 andinterconnects 1326 corresponding to the through-wafer interconnects 224of the imaging dies 226-described above. The flexor units 1320illustrated in FIG. 13A are arranged in a pattern corresponding to thepattern of the imaging dies 220 on the assembly 200 such that eachimaging die 220 can be attached to a corresponding flexor unit 1320.

FIGS. 13B and 13C are cross-sectional views illustrating subsequentstages of constructing imager devices in accordance with aspects of thisembodiment of the invention. FIG. 13B illustrates a stage at which theassembly 200 has been mounted to the backing member 1300 such that theimage sensors 221 are aligned-with the shaped surfaces 1322 and thethrough-wafer interconnects 224 are engaged with the interconnects 1326.The image sensors 221 are accordingly aligned with molding sites definedby the shaped surfaces 1322. FIG. 13C illustrates a later stage at whichthe imaging dies 220 have been shaped to at least generally conform theimage sensors 221 to the shaped surfaces 1322. The imaging dies 220,more specifically, can be bent in discrete regions of the substrate 212to conform the image sensors 221 to the shaped surfaces 1322. In oneembodiment, the shaped surfaces 1322 and openings 1324 comprise vacuumcups, and the discrete regions of the substrate 212 are drawn againstthe shaped surfaces 1322 by reducing the pressure in the vacuum cupsuntil the substrate 212 adequately conforms to the shaped surfaces 1322.The backside of the substrate 212 or the shaped surfaces 1322 can becoated with an adhesive to bond the substrate 212 to the shaped surfaces1322. The openings 1324 can then be filled with a plug 1330 to protectthe backside of the substrate 212.

FIG. 13D is a cross-sectional view illustrating a later stage of forminga plurality of microelectronic imager devices 1340 in accordance with anaspect of this embodiment of the invention. At the stage illustrated inFIG. 13D, the cover 240, substrate 212, and plate 1310 are cut in theregions between the imaging dies 220 to separate the individual imagerdevices 1340 from one another. The procedure illustrated in FIG. 13D canhave one or more cutting steps to cut through the various components.For example, the cover 240 and substrate 212 can be cut using one typeof blade set, and the plate 1310 can be cut using a different blade set.In other embodiment, the cover 240 and a significant portion of thespacer 230 can be cut with a first blade, the substrate 212 can be cutwith a second blade that is thinner than the first blade, and the plate1310 can be cut with the first blade used to cut the cover 240 or athird blade that is different from the first and second blades. Themulti-step cutting process may be advantageous because the cover 240,spacer 230, and plate 1310 can be cut with relatively thick blades thatproduce a large gap, and then the substrate 212 can be cut with arelatively thin blade to mitigate the kerf through the substrate 212.

Several embodiments of the method illustrated in FIGS. 13A–13D canprecisely shape the substrate 212 to impart the desired curvature orother shape to the image sensors 221. For example, because the shapedsurfaces can be formed to precise contours using molding, etching,embossing or other processes, the backing member 1300 provides precisemolding units to shape the image sensors 221. This is expected tofurther enhance the ability to use curved image sensors inmicroelectronic imager devices.

Several embodiments of the method illustrated in FIGS. 13A–13D are alsoexpected to provide high throughput and low cost processes for formingcurved image sensors. First, because the image sensors 221 can be shapedat the wafer level, all of the image sensors can be processed at leastsubstantially simultaneously to enhance the throughput of the method.Second, the backing member 1300 is relatively inexpensive to form using(a) known molding or embossing processes to form the shaped surfaces1322, and (b) known mechanical or laser processes to form the openings1324. Therefore, the processes illustrated in FIGS. 13A–D provideinexpensive, high throughput procedures for manufacturingmicroelectronic imager devices.

FIGS. 14A and 14B are cross-sectional views illustrating anotherembodiment for forming microelectronic imager devices in accordance withthe invention. FIG. 14A illustrates a different embodiment of a backingmember 1400 having a plate 1410 and a plurality of shaped surfaces 1422defined by raised or curved convex areas along the plate 1410. Thebacking member 1400 can further include a plurality of interconnects1326 as described above with reference to FIG. 13A. FIG. 14B illustratesa subsequent stage of this method in which the assembly 200 has beenattached to the backing member 1400 such that the image sensors 221 arealigned with corresponding shaped surfaces 1422. In this embodiment, theconvex shaped surfaces 1422 bend the image sensors 221 toward the cover240. The assembly 200 can be adhered to the backing member 1400 with anadhesive, and the assembled workpiece can be cut to separate individualmicroelectronic imager devices from one another as explained above.

One concern of bending the substrate 212 to form curved image sensors isthat the curvature of the image sensors should be selected to avoidexcessive “dark current” that may affect the performance of the devices.Dark current in an image sensor is the background electrical current inthe device when no radiation is present. In the case of an image sensorthat detects radiation in the visible spectrum, it is the backgroundelectrical current in the device when it is dark (i.e., no light ispresent). Dark current can be caused by defects in the silicon crystal(e.g., the epitaxial layer), and bending the substrate stresses theepitaxial layer and may produce defects. FIG. 15 is a graph illustratingthe potential need to mitigate the extent of dark current. Morespecifically, FIG. 15 is a bar graph in which a first bar 1500illustrates the actual signal strength of light impinging upon an imagesensor and a second bar 1510 illustrates the dark current signal. Thenet dynamic range is a difference between the actual signal strength1500 and the dark current signal 1510. To obtain an adequate net dynamicrange for an image sensor, the dark current signal 1510 should belimited to avoid excessive noise.

Several embodiments of the curved image sensors described above withreference to FIGS. 2–14B may be shaped to mitigate the extent of darkcurrent. FIG. 16 is a partial cross-sectional view of the plate 1310described above with reference to FIG. 13A. In this embodiment, theplate 1310 is configured to limit the curvature of the substrate 212 toan amount that does not produce excessive dark current and/or excessivenon-uniformities in the dark current. To accomplish this, the shapedsurfaces in the plate 1310 have precise curvatures that may bespecifically selected according to the type of image sensor. Forexample, high definition cameras have relatively low tolerances for darkcurrent such that a shaped surface 1322 a may have a shallow orrelatively large radius of curvature C₁ to limit the curvature of theimage sensor. Conversely, temperature sensors or other sensors havehigher tolerances such that a shaped surface 1322 b may have a deeper orrelatively smaller radius of curvature C₂. It will be appreciated thatthe difference between the curvatures C₁ and C₂ in FIG. 16 areexaggerated for purposes of illustration.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. For example, the substrate 212 canhave patterns of trenches or other voids etched on the front side 214and/or the backside 216 to preferentially direct the flexure of thesubstrate 212 using any of the embodiments described above with respectto FIGS. 5–14B. Similarly, ridges or other protrusions can be formed onthe substrate 212 in lieu of or in addition to voids to preferentiallydirect the flexure of the substrate. Also, aspects of the inventiondescribed in the particular embodiments may be combined with each otheror eliminated in other embodiments. Accordingly, the invention is notlimited except as by the appended claims.

1. A microelectronic imager device, comprising: an imaging diecomprising a substrate having a front side and a backside, amicroelectronic image sensor comprising a face located at the front sideof the substrate, integrated circuitry connected to the image sensor,and backside interconnects electrically coupled to the integratedcircuitry; and a flexor unit comprising a portion of a backing member,the portion of the backing member comprising backing memberinterconnects electrically connected to the backside interconnects and ashaped surface having a non-planar contour, wherein a portion of thebackside of the substrate is attached to the shaped surface of thebacking member such that the face of the image sensor is curved.
 2. Theimager device of claim 1 wherein the shaped surface comprises a curvedsurface having a radius of curvature configured to limit dark current inthe image sensor.
 3. The imager device of claim 1 wherein the substrateconforms to the shaped surface of the backing member.
 4. The imagerdevice of claim 1, further comprising a standoff attached to the frontside of the substrate and a cover attached to the standoff.
 5. Theimager device of claim 1 wherein the shaped surface comprises a curveddepression in the backing member and the backing member furthercomprises an opening in the depression to form a vacuum cup, and whereinthe substrate conforms to the vacuum cup.
 6. A microelectronic imagerdevice assembly, comprising: a substrate comprising a front side, abackside, and a plurality of imaging dies arranged in a die pattern,wherein the imaging dies comprise microelectronic image sensors locatedat the front side of the substrate, integrated circuitry electricallycoupled to the image sensors, and backside interconnects electricallycoupled to the integrated circuitry; and a backing member including aplurality of curved surfaces arranged in the die pattern and backingmember interconnects electrically connected to the backsideinterconnects of the imaging dies, wherein individual curved surfacesare aligned with a corresponding image sensor and attached to thebackside of the substrate.
 7. The imager device assembly of claim 6wherein the curved surfaces have a radius of curvature configured tolimit dark current in the image sensor.
 8. The imager device of claim 6wherein the substrate conforms to the curved surfaces such that theimage sensors have a curvature corresponding to the curved surfaces. 9.The imager device of claim 6, further comprising standoffs attached tothe front side of the substrate and a cover attached to the standoffs.10. The imager device of claim 6 wherein the curved surfaces comprisedepressions in the backing member and the backing member furthercomprises one or more openings in the depressions to form vacuum cups,and wherein the substrate conforms to the vacuum cups.
 11. A method ofmanufacturing microelectronic imager devices, comprising: providing asubstrate comprising a front side, a backside and a plurality of imagingdies arranged in a die pattern, wherein the imaging dies comprisemicroelectronic image sensors located at the front side of thesubstrate, integrated circuitry electrically coupled to the imagesensors, and backside interconnects electrically connected to theintegrated circuitry; providing a backing member comprising a pluralityof curved surfaces arranged in the die pattern and backing memberinterconnects; and attaching the curved surfaces to the backside of thesubstrate such that the curved surfaces are aligned with correspondingimage sensors and the backside interconnects are electrically connectedto the backing member interconnects.
 12. The method of claim 11 whereinthe curved surfaces are depressions having a curvature corresponding toa desired curvature for the image sensors, and wherein attaching thecurved surfaces to the backside of the substrate comprises locallybending the substrate until the backside of the substrate conforms tothe depressions.
 13. The method of claim 11 wherein the curved surfacesdefine cups and the backing member further comprises openings alignedwith the cups, and wherein attaching the curved surfaces to the backsideof the substrate comprises drawing a vacuum through the openings thatdraws the backside of the substrate into the cups.
 14. The method ofclaim 13, further comprising adhering the backside of the substrate tothe curved surfaces.
 15. The method of claim 11, further comprisingcutting the substrate and the backing member after attaching the curvedsurfaces to the backside of the substrate to separate the imaging diesfrom each other.
 16. The method of claim 11, further comprising formingstandoffs on the front side of the substrate, attaching a cover to thestandoffs, and separating the imaging dies from each other by cuttingthe cover, the substrate and the backing member.
 17. The method of claim11 wherein the curved surfaces are depressions.
 18. The method of claim11 wherein the curved surfaces are raised features.